Sorbents for coal combustion

ABSTRACT

Sorbent compositions containing halogen and calcium are added to coal to mitigate the release of sulfur and/or other harmful elements, including mercury, into the environment during combustion of coal containing natural levels of mercury.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. Ser. No.13/343,491 filed on Jan. 4, 2012 (U.S. Pat. No. 8,313,323 issue dateNov. 20, 2012); which is a continuation of U.S. Ser. No. 13/169,187filed on Jun. 27, 2011 (now U.S. Pat. No. 8,114,368 which issued Feb.14, 2012); which is a continuation of U.S. Ser. No. 12/839,154 filed onJul. 19, 2010 (now U.S. Pat. No. 7,988,939 which issued Aug. 2, 2011);which is a continuation of U.S. Ser. No. 12/705,196 filed on Feb. 12,2010 (now U.S. Pat. No. 7,776,301 which issued Aug. 17, 2010); which isa continuation of U.S. Ser. No. 12/351,191 filed Jan. 9, 2009 (now U.S.Pat. No. 7,674,442 which issued on Mar. 9, 2010) which is a continuationof U.S. Ser. No. 11/377,528 filed Mar. 16, 2006 (now U.S. Pat. No.7,507,083 which issued Mar. 24, 2009), which claims the benefit of U.S.Provisional Application 60/662,911 filed Mar. 17, 2005, the fulldisclosures of which are hereby incorporated by reference.

INTRODUCTION

The invention provides compositions and methods for reducing the levelsof mercury emitted into the atmosphere upon burning of mercurycontaining fuels such as coal. In particular, the invention provides foraddition of various halogen and other sorbent compositions into the coalburning system during combustion.

Significant coal resources exist around the world, that are capable ofmeeting large portions of the world's energy needs into the next twocenturies. High sulfur coal is plentiful, but requires remediation stepsto prevent excess sulfur from being released into the atmosphere uponcombustion. In the United States, low sulfur coal exists in the form oflow BTU value coal in the Powder River basin of Wyoming and Montana, inlignite deposits in the North Central region of North and South Dakota,and in lignite deposits in Texas. But even when coals contain lowsulfur, they still contain non-negligible levels of elemental andoxidized mercury.

Unfortunately, mercury is at least partially volatilized upon combustionof coal. As a result, the mercury tends not to stay with the ash, butrather becomes a component of the flue gases. If remediation is notundertaken, the mercury tends to escape from the coal burning facility,leading to environmental problems. Some mercury today is captured byutilities, for example in wet scrubber and SCR control systems. However,most mercury is not captured and is therefore released through theexhaust stack.

In the United States, the Clean Air Act Amendments of 1990 contemplatedthe regulation and control of mercury. A mercury study in the report toCongress in 1997 by the Environmental Protection Agency (EPA) furtherdefined the bounds of mercury release from power plants in the UnitedStates. In December 2000, the EPA decided to regulate mercury, and havepublished proposed clean air mercury rules in January and March of 2004.A set of regulations for required mercury reduction from US coal burningplants has now been promulgated by the United States EnvironmentalProtection Agency.

In addition to wet scrubber and SCR control systems that tend to removemercury partially from the flue gases of coal combustion, other methodsof control have included the use of activated carbon systems. Use ofsuch systems tends to be associated with high treatment costs andelevated capital costs. Further, the use of activated carbon systemsleads to carbon contamination of the fly ash collected in exhaust airtreatments such as the bag house and electrostatic precipitators.

Mercury emissions into the atmosphere in the United States areapproximately 50 tons per year. A significant fraction of the releasecomes from emissions from coal burning facilities such as electricutilities. Mercury is a known environmental hazard and leads to healthproblems for both humans and non-human animal species. To safeguard thehealth of the public and to protect the environment, the utilityindustry is continuing to develop, test, and implement systems to reducethe level of mercury emissions from its plants. In combustion ofcarbonaceous materials, it is desirable to have a process whereinmercury and other undesirable compounds are captured and retained afterthe combustion phase so that they are not released into the atmosphere.

SUMMARY

Processes and compositions are provided for decreasing emissions ofmercury upon combustion of fuels such as coal. Various sorbentcompositions are provided that contain components that reduce the levelof mercury and/or sulfur emitted into the atmosphere upon burning ofcoal. In various embodiments, the sorbent compositions are addeddirectly to the fuel before combustion; are added partially to the fuelbefore combustion and partially into the flue gas post combustion zone;or are added completely into the flue gas post combustion zone. Inpreferred embodiments, the sorbent compositions comprise a source ofhalogen and preferably a source of calcium. Among the halogens, iodineand bromine are preferred. In various embodiments, inorganic bromidesmake up a part of the sorbent compositions.

In various embodiments mercury sorbent compositions containing bromineor iodine are added to the fuel as a powder or a liquid prior tocombustion. Alternatively, the sorbent compositions containing halogensuch as bromine and iodine are injected into the flue gas at a pointafter the combustion chamber where the temperature is higher than about1500° F. (about 800° C.).

In preferred embodiments, the sorbent compositions further contain othercomponents, especially a source of calcium. Thus, in one embodiment, theinvention provides for singular and multiple applications ofmulti-element oxidizers, promoters, and sorbents to coal prior to and/orafter combustion in a furnace. In various embodiments, the components ofthe sorbent compositions develop ceramic characteristics upon combustionand subsequent calcination of the components with the carbonaceousmaterials. In various embodiments, use of the sorbent compositionsreduces mercury emissions by capturing and stabilizing oxidized andelemental mercury with multiple-element remediation materials such ascalcium oxides, calcium bromides, other calcium halogens, as well asoxides of silicon, aluminum, iron, magnesium, sodium, and potassium.

In preferred embodiments, mercury emissions from coal burning facilitiesare reduced to such an extent that 90% or more of the mercury in thecoal is captured before release into the atmosphere. The mercuryremediation processes can be used together with sorbent compositions andother processes that remove sulfur from the combustion gas steam. Thusin preferred embodiments, significant sulfur reduction is achieved alongwith 90% plus reduction of mercury emissions.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

DESCRIPTION

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

In various embodiments, the invention provides compositions and methodsfor reducing emissions of mercury that arise from the combustion ofmercury containing fuels such as coal. Systems and facilities that burnfuels containing mercury will be described with particular attention tothe example of a coal burning facility such as used by electricalutilities. Such facilities generally have some kind of feeding mechanismto deliver the coal into a furnace where the coal is burned orcombusted. The feeding mechanism can be any device or apparatus suitablefor use. Non-limiting examples include conveyer systems, screw extrusionsystems, and the like. In operation, a mercury-containing fuel such ascoal is fed into the furnace at a rate suitable to achieve the outputdesired from the furnace. Generally, the output from the furnace is usedto boil water for steam to provide direct heat, or else the steam isused to turn turbines that eventually result in the production ofelectricity.

The coal is fed into the furnace and burned in the presence of oxygen.Typical flame temperatures in the combustion temperature are on theorder of 2700° F. to about 3000° F. After the furnace or boiler wherethe fed fuel is combusted, the facility provides convective pathways forthe combustion gases, which for convenience are sometimes referred to asflue gases. Hot combustion gases and air move by convection away fromthe flame through the convective pathway in a downstream direction(i.e., downstream in relation to the fireball. The convection pathway ofthe facility contains a number of zones characterized by the temperatureof the gases and combustion products in each zone. Generally, thetemperature of the combustion gas falls as it moves in a directiondownstream from the fireball. The combustion gases contain carbondioxide as well as various undesirable gases containing sulfur andmercury. The convective pathways are also filled with a variety of ashwhich is swept along with the high temperature gases. To remove the ashbefore emission into the atmosphere, particulate removal systems areused. A variety of such removal systems can be disposed in theconvective pathway such as electrostatic precipitators and a bag house.In addition, chemical scrubbers can be positioned in the convectivepathway. Additionally, there may be provided various instruments tomonitor components of the gas such as sulfur and mercury.

From the furnace, where the coal is burning at a temperature ofapproximately 2700° F.-3000° F., the fly ash and combustion gases movedownstream in the convective pathway to zones of ever decreasingtemperature. Immediately downstream of the fireball is a zone withtemperature less that 2700° F. Further downstream, a point is reachedwhere the temperature has cooled to about 1500° F. Between the twopoints is a zone having a temperature from about 1500 to about 2700° F.Further downstream, a zone of less than 1500° F. is reached, and so on.Further along in the convective pathway, the gases and fly ash passthrough lower temperature zones until the baghouse or electrostaticprecipitator is reached, which typically has a temperature of about 300°F. before the gases are emitted up the stack.

In various embodiments, the process of the present invention calls forthe application of a mercury sorbent

directly to a fuel such as coal before combustion (addition“pre-combustion”);

directly into the gaseous stream after combustion in a temperature zoneof between 2700° F. and 1500° F. (addition “post-combustion); or

in a combination of pre-combustion and post-combustion additions.

In various embodiments, oxidized mercury from combustion reports to thebag house or electrostatic precipitator and becomes part of the overallash content of the coal burning plant. Heavy metals in the ash do notleach below regulatory levels.

In various embodiments, mercury emissions from the coal burning facilityare monitored. Depending on the level of mercury in the flue gas priorto emission from the plant, the amount of sorbent composition added ontothe fuel per- and/or post-combustion is raised, lowered, or ismaintained unchanged. In general, it is desirable to remove as high alevel of mercury as is possible. In typical embodiments, mercury removalof 90% and greater are achieved, based on the total amount of mercury inthe coal. This number refers to the mercury removed from the flue gasesso that mercury is not released through the stack into the atmosphere.To minimize the amount of sorbent added into the coal burning process soas to reduce the overall amount of ash produced in the furnace, it isdesirable in many environments to use the measurements of mercuryemissions to reduce the sorbent composition rate of addition to onewhich will achieve the desired mercury reduction without adding excessmaterial into the system.

Thus in one embodiment, a method is provided for burning coal to reducethe amount of mercury released into the atmosphere. The method involvesfirst applying a sorbent composition comprising a halogen compound ontothe coal. The coal is then delivered into the furnace of a coal burningplant. The coal containing the sorbent composition is then combusted inthe furnace to produce ash and combustion gases. The combustion gasescontain mercury, sulfur and other components. To accomplish a desiredreduction of mercury in the combustion gases in order to limit releaseinto the atmosphere, the mercury level in the combustion gases ispreferably monitored by measuring the level analytically. In preferredembodiments, the amount of the sorbent composition applied onto the coalbefore composition is adjusted depending on the value of the mercurylevel measured in the combustion gases.

In another embodiment, a mercury sorbent is added into the coal burningsystem after combustion in a region having a temperature from about1500° F. to 2700° F. (about 815° C. to 1482° C.). A method is providedfor reducing the level of mercury released into the atmosphere uponcombustion of coal that contains mercury. The combustion is carried outin a coal burning system containing a furnace and a convective pathwayfor the combustion gases. The method involves burning the coal in thefurnace, and injecting a sorbent containing a halogen into theconvective pathway at a point where the combustion gases are at atemperature of 1500° F. to 2700° F. If desired, the level of mercury inthe gases escaping the facility is monitored and measured. Depending onthe level of mercury escaping from the facility, reflected in the valuedetermined by monitoring, the rate of addition of the mercury sorbentcan be increased, decreased, or maintained unchanged. In a furtherembodiment, a mercury sorbent containing a halogen can be both appliedto the coal prior to combustion and injected into the convective pathwayat a point where the combustion gases are at a temperature of 1500° F.to 2700° F.

Sorbent composition comprising a halogen compound contains one or moreorganic or inorganic compounds containing a halogen. Halogens includechlorine, bromine, and iodine. Preferred halogens are bromine andiodine. The halogen compounds noted above are sources of the halogens,especially of bromine and iodine. For bromine, sources of the halogeninclude various inorganic salts of bromine including bromides, bromates,and hypobromites. In various embodiments, organic bromine compounds areless preferred because of their cost or availability. However, organicsources of bromine containing a suitably high level of bromine areconsidered within the scope of the invention. Non-limiting examples oforganic bromine compounds include methylene bromide, ethyl bromide,bromoform, and carbonate tetrabromide. Non-limiting sources of iodineinclude hypoiodites, iodates, and iodides, with iodides being preferred.

When the halogen compound is an inorganic substituent, it is preferablya bromine or iodine containing salt of an alkali metal or an alkalineearth element. Preferred alkali metals include lithium, sodium, andpotassium, while preferred alkaline earth elements include beryllium,magnesium, and calcium. Of halogen compounds, particularly preferred arebromides and iodides of alkaline earth metals such as calcium.

The sorbent composition containing the halogen is provided in the formof a liquid or of a solid composition. When it is a liquid composition,the sorbent composition comprises preferably an aqueous solution of abromine or iodine compound as described above. The methods of theinvention that reduce the level of mercury released into the atmosphereupon combustion of coal involve applying the sorbent composition, in theform of either a liquid or a solid composition, into the coal burningprocess. In one embodiment, the sorbent composition is added to the coalprior to combustion, while in another the sorbent composition isinjected into the convective pathway of the coal burning facility in azone having a temperature of 1500° F. to 2700° F. In variousembodiments, sorbent addition can take place both pre-combustion andpost-combustion. In a preferred embodiment, an aqueous sorbentcontaining a halogen is sprayed onto the coal pre-combustion and thecoal enters the furnace still wet with water.

In various embodiments, liquid mercury sorbent comprises a solutioncontaining 5 to 60% by weight of a soluble bromine or iodine containingsalt. Non-limiting examples of preferred bromine and iodine saltsinclude calcium bromide and calcium iodide. In various embodiments,liquid sorbents contain 5-60% by weight of calcium bromide and/orcalcium iodide. For efficiency of addition to the coal prior tocombustion, in various embodiments it is preferred to add mercurysorbents having as high level of bromine or iodine compound as isfeasible. In a non-limiting embodiment, the liquid sorbent contains 50%or more by weight of the halogen compound, such as calcium bromide orcalcium iodide.

In various embodiments, the sorbent compositions containing a halogencompound further contain a nitrate compound, a nitrite compound, or acombination of nitrate and nitrite compounds. Preferred nitrate andnitrite compounds include those of magnesium and calcium, preferablycalcium. Thus, in a preferred embodiment, the mercury sorbentcomposition contains calcium bromide. Calcium bromide can be formulatedwith other components such as the nitrates and nitrites discussed aboveand to either a powder sorbent composition or a liquid sorbentcomposition. The powder or liquid sorbent compositions containinghalogen are added on to the coal pre-combustion, injected into theconvective pathways of the coal burning facility in a zone having atemperature of about 1500° F. to about 2700° F. or a combination of thetwo.

The mercury sorbent compositions containing a halogen compoundpreferably further comprise a source of calcium. Non-limiting examplesof calcium sources include calcium oxides, calcium hydroxides, calciumcarbonate, calcium bicarbonate, calcium sulfate, calcium bisulfate,calcium nitrate, calcium nitrite, calcium acetate, calcium citrate,calcium phosphate, calcium hydrogen phosphate, and calcium minerals suchas apatite and the like. Preferred sources of calcium include calciumhalides, such as calcium bromide, calcium chloride, and calcium iodide.Organic calcium compounds can also be used. Non-limiting examplesinclude calcium salts of carboxylic acids, calcium alkoxylates, andorganocalcium compounds. As with the halogen compounds above, in variousembodiments, the organic calcium compounds tend to be less preferredbecause of expense and availability.

In addition to the mercury sorbent composition added into the systembefore or after combustion, a sulfur sorbent composition may be addedalong with the mercury sorbent. Thus, in preferred embodiments, methodsare provided for reducing both sulfur and mercury emissions in the fluegas upon combustion of coal containing sulfur and mercury. In apreferred embodiment, a method involves applying a first sorbentcomposition and a second sorbent composition into the system. One of thefirst and second sorbent compositions is added to the coal prior tocombustion and the other is injected into the coal burning system in azone of the convective pathway downstream of the burning chamber,preferably where the temperature is in the range of between 1500° F. to2700° F. The first sorbent composition preferably contains a halogencomponent and is added at level effective to reduce mercury in thecombustion gases. The second sorbent composition contains at least acalcium component and is added at level effective to reduce sulfur inthe combustion gases.

In the embodiments of the previous paragraph, the first sorbentcomposition containing the halogen component comprises a halogencompound such as the preferred bromine and iodine compounds describedabove. The second sorbent composition contains calcium in a formsuitable for the reduction of sulfur emissions from the burning coalsystem. The second sorbent composition containing a calcium componentpreferably contains calcium in a minimum molar amount of 1:1 based onthe molar amount of sulfur present in the coal. Preferably, the level ofcalcium added to the system with the second sorbent composition is nogreater than 3:1 with respect to moles of sulfur in the coal. Treatmentat higher levels of calcium tends to waste material and risks blindingoff the furnace, thereby impeding the combustion process and loading theparticulate control system.

Essentially, it is desired to add the calcium-containing sulfur sorbentat a level effective to remove sulfur from the flue gases of the burningcoal, but not in an over abundant amount that would lead to productionof excess ash. The second sorbent composition containing a calciumcomponent can contain any of the inorganic or organic calcium compoundsnoted above. In addition, various industrial products contain calcium ata suitable level, such as cement kiln dust, lime kiln dust, Portlandcement, and the like. In various embodiments, the calcium-containingsulfur sorbent contains a calcium powder such as those listed, alongwith an aluminosilicate clay such as montmorillonite or kaolin. Thecalcium containing sulfur sorbent composition preferably containssufficient SiO₂ and Al₂O₃ to form a refractory-like mixture with calciumsulfate produced by combustion, such that the calcium sulfate is handledby the particle control system of the furnace. In preferred embodiments,the calcium containing sulfur absorbent contains a minimum of 2% silicaand 2% alumina.

In a preferred embodiment, a mercury sorbent composition containingcalcium and bromine is applied to the coal. In various embodiments, thesorbent composition contains calcium bromide. Alternatively, theabsorbent composition contains a bromine compound other than calciumbromide and a calcium compound other than calcium bromide. Non-limitingexamples of sources of calcium include calcium bromide, calcium nitrite,Portland cement, calcium oxide, calcium hydroxide and calcium carbonate.Then the coal containing the calcium and bromine sorbent composition isburned to produce ash and combustion gases. Desirably, the level ofmercury in the combustion gases is measured and monitored. The level ofbromine added to the coal by way of the sorbent composition is thenadjusted up or down or left unchanged, depending on the level of mercurymeasured in the combustion gases. In various embodiments, the methodfurther provides for measuring a level of sulfur in the combustion gasesand adjusting the level of calcium added onto the coal based on thelevel of sulfur measured. In preferred embodiments, mercury emissionsinto the environment from the coal burning facility are reduced by 90%or more. As used in this application, a mercury reduction of 90% or moremeans at least 90% of the mercury in the coal being burned is capturedto prevent its release into the atmosphere. Preferably, a sufficientamount of bromine is added onto the coal prior to combustion to reducethe mercury emissions into the environment by 90% or more.

In one aspect, the invention involves reducing the level of mercuryemitted into the atmosphere from facilities that burn fuels containingmercury. A commercially valuable embodiment is use of the invention toreduce mercury emissions from coal burning facilities to protect theenvironment and comply with government regulations and treatyobligations. Much of the following discussion will refer to coal as thefuel; it is to be understood that the description of coal burning is forillustrative purposes only and the invention is not necessarily to belimited thereby.

In various embodiments, the methods of the invention involve adding amercury sorbent into the fuel or coal burning system at treatment levelssufficient to cause a desired lowering of the levels of mercury escapingfrom the facility into the atmosphere upon combustion of the fuel.Suitable mercury sorbents are described above. In a preferredembodiment, the mercury sorbents contain a source of bromine and/oriodine, preferably in the form of inorganic bromide or iodide salts asdiscussed above.

In one embodiment, the mercury sorbent composition is added onto coalprior to its combustion. The coal is particulate coal, and is optionallypulverized or powdered according to conventional procedures. The sorbentcomposition is added onto the coal as a liquid or as a solid. Generally,solid sorbent compositions are in the form of a powder. If the sorbentis added as a liquid (usually as a solution of one or more bromine oriodine salts in water), in one embodiment the coal remains wet when fedinto the burner. The sorbent composition can be added onto the coalcontinuously at the coal burning facility by spraying or mixing onto thecoal while it is on a conveyor, screw extruder, or other feedingapparatus. In addition or alternatively, the sorbent composition may beseparately mixed with the coal at the coal burning facility or at thecoal producer. In a preferred embodiment, the sorbent composition isadded as a liquid or a powder to the coal as it is being fed into theburner. For example, in a preferred commercial embodiment, the sorbentis applied into the pulverizers that pulverize the coal prior toinjection. If desired, the rate of addition of the sorbent compositioncan be varied to achieve a desired level of mercury emissions. In oneembodiment, the level of mercury in the flue gases is monitored and thelevel of sorbent addition adjusted up or down as required to maintainthe desired mercury level.

Mercury levels can be monitored with conventional analytical equipmentusing industry standard detection and determination methods. In oneembodiment, monitoring is conducted periodically, either manually orautomatically. In a non-limiting example, mercury emissions aremonitored once an hour to ensure compliance with government regulations.To illustrate, the Ontario Hydro method is used. In this known method,gases are collected for a pre-determined time, for example one hour.Mercury is precipitated from the collected gases, and the level isquantitated using a suitable method such as atomic absorption.Monitoring can also take more or less frequently than once an hour,depending on technical and commercial feasibility. Commercial continuousmercury monitors can be set to measure mercury and produce a number at asuitable frequency, for example once every 3-7 minutes. In variousembodiments, the output of the mercury monitors is used to control therate of addition of mercury sorbent. Depending on the results ofmonitoring, the rate of addition of the mercury sorbent is adjusted byeither increasing the level of addition; decreasing it, or leaving itunchanged. To illustrate, if monitoring indicates mercury levels arehigher than desired, the rate of addition of sorbent is increased untilmercury levels return to a desired level. If mercury levels are atdesired levels, the rate of sorbent addition can remain unchanged.Alternatively, the rate of sorbent addition can be lowered untilmonitoring indicates it should be increased to avoid high mercurylevels. In this way, mercury emission reduction is achieved andexcessive use of sorbent (with concomitant increase of ash) is avoided.

Mercury is monitored in the convective pathway at suitable locations. Invarious embodiments, mercury released into the atmosphere is monitoredand measured on the clean side of the particulate control system.Mercury can also be monitored at a point in the convective pathwayupstream of the particulate control system. Experiments show that asmuch as 20 to 30% of the mercury in coal is captured in the ash and notreleased into the atmosphere when no mercury sorbent is added. Additionof mercury sorbents according to the invention raises the amount ofmercury capture (and thus reduces the amount of mercury emissions) to90% or more.

Alternatively or in addition, a mercury sorbent composition is insertedor injected into the convective pathway of the coal burning facility toreduce the mercury levels. Preferably, the sorbent composition is addedinto a zone of the convective pathway downstream of the fireball (causedby combustion of the coal), which zone has a temperature above about1500° F. and less than the fireball temperature of 2700-3000° F. Invarious embodiments, the temperature of sorbent is above about 1700° F.The zone preferably has a temperature below about 2700° F. In variousembodiments, the injection zone has a temperature below 2600° F., belowabout 2500° F. or below about 2400° F. In non-limiting examples, theinjection temperature is from 1700° F. to 2300° F., from 1700° F. to2200° F., or from about 1500° F. to about 2200° F. As withpre-combustion addition, the sorbent can be in the form of a liquid or asolid (powder), and contains an effective level of a bromine or iodinecompound. In various embodiments, the rate of addition of sorbent intothe convective pathway is varied depending on the results of mercurymonitoring as described above with respect to pre-combustion addition ofsorbent.

In preferred embodiments, sorbent composition is added more or lesscontinuously to the coal before combustion and/or to the convectivepathway in the 1500° F.-2700° F. zone as described above. In variousembodiments, automatic feedback loops are provided between the mercurymonitoring apparatus and the sorbent feed apparatus. This allows for aconstant monitoring of emitted mercury and adjustment of sorbentaddition rates to control the process.

Along with the mercury sorbent, a sulfur sorbent is preferably added tocontrol the release of sulfur into the environment. In variousembodiments, the sulfur sorbent is added into the coal burning system atthe same places the mercury sorbent is added. The sulfur sorbent canalso be added at other places, depending on technical feasibility. Invarious embodiments, the components of the mercury sorbent and sulfurare combined into a single sorbent added to the coal or injected intothe convective pathway. The sorbents, either separately or combined, areadded in the form of a liquid or a solid. Solid compositions are usuallyin the form of a powder.

The sulfur sorbent preferably contains calcium at a level at leastequal, on a molar basis, to the sulfur level present in the coal beingburned. As a rule of thumb, the calcium level should be no more thanabout three times, on a molar basis, the level of sulfur. The 1:1 Ca:Slevel is preferred for efficient sulfur removal, and the upper 3:1 ratiois preferred to avoid production of excess ash from the combustionprocess. Treatment levels outside the preferred ranges are also part ofthe invention. Suitable sulfur sorbents are described, for example, inco-owned provisional application 60/583,420, filed Jun. 28, 2004, thedisclosure of which is incorporated by reference.

Preferred sulfur sorbents include basic powders that contain calciumsalts such as calcium oxide, hydroxide, and carbonate. Other basicpowders containing calcium include portland cement, cement kiln dust,and lime kiln dust. In various embodiments, the sulfur sorbent alsocontains an aluminosilicate clay, montmorillonite, and/or kaolin.Preferably the sulfur sorbent contains suitable levels of silica andalumina (in a preferred embodiment, at least about 2% by weight of each)to form refractory materials with calcium sulfate formed by combustionof sulfur-containing coal. Silica and alumina can be added separately oras components of other materials such as Portland cement. In variousembodiments, the sulfur sorbent also contains a suitable level ofmagnesium as MgO, contributed for example by dolomite or as a componentof portland cement. In a non-limiting example, the sulfur sorbentcontains 60-71% CaO, 12-15% SiO₂, 4-18% Al₂O₃, 1-4% Fe₂O₃, 0.5-1.5% MgO,and 0.1-0.5% Na₂O.

The mercury and sulfur sorbents can be added together or separately. Forconvenience, the components of the mercury and sulfur sorbents can becombined before addition onto the coal or injection into the convectivepathways. In a preferred embodiment, the mercury sorbent containscalcium in addition to a source of halogen. In various embodiments, themercury sorbent composition further comprises components that alsoreduce sulfur. The invention provides for addition of various sorbentcompositions into the coal burning system to reduce emissions of mercuryand, preferably, also of sulfur.

In various embodiments, sulfur and mercury sorbents are addedseparately. For example, a mercury sorbent is added to the coalpre-combustion and a sulfur sorbent is added post-combustion.Alternatively, a mercury sorbent is added post-combustion, while asulfur sorbent is added pre-combustion. No matter the mode of addition,in a preferred embodiment the rate of addition of the various sorbentsis adjusted as required on the basis of values of emitted sulfur andmercury determined by monitoring.

Mercury and sulfur sorbents are added at levels required to achieve thedesired amount of reduced emissions. Preferred mercury reduction is 70%or more, preferably 80% or more and more preferable 90% or more, basedon the total mercury in the coal being burned. On a weight basis, themercury sorbent is generally added at a level of about 0.01 to 10% basedon the weight of the coal. Preferred ranges include 0.05 to 5% and 0.1to 1% by weight. The treat level varies depending on the content ofhalogen in the sorbent and the desired level of mercury emissions to beachieved. A level of 0.3% is suitable for many embodiments. In variousembodiments, the initial treat level is adjusted up or down as requiredto achieve a desired emission level, based on monitoring as discussedabove. The sorbent can be added in batch or continuously. In embodimentswith continuous addition of sorbent, the treat levels are based on thefeed rate of the coal being burned. Where the sorbent is added in batch,such as at the coal producer or at a separate mixing facility, the treatlevel is based on the weight of the coal being treated. In a preferredembodiment, the rate of addition or the treat level is adjusted based ona determination of emitted levels of mercury.

Likewise, sulfur sorbent is added at a level or rate satisfactory forreducing the level of emitted sulfur to an acceptable or desired level.In various embodiments, about 1 to 9% by weight of sulfur sorbent isadded. The level or rate can be adjusted if desired based on the levelof emitted sulfur determined by monitoring.

In preferred embodiments, mercury and sulfur are monitored usingindustry standard methods such as those published by the AmericanSociety for Testing and Materials (ASTM) or international standardspublished by the International Standards Organization (ISO). Anapparatus comprising an analytical instrument is preferably disposed inthe convective pathway downstream of the addition points of the mercuryand sulfur sorbents. In a preferred embodiment, a mercury monitor isdisposed on the clean side of the particulate control system. In variousembodiments, a measured level of mercury or sulfur is used to providefeedback signals to pumps, solenoids, sprayers, and other devices thatare actuated or controlled to adjust the rate of addition of a sorbentcomposition into the coal burning system. Alternatively or in addition,the rate of sorbent addition can be adjusted by a human operator basedon the observed levels of mercury and/or sulfur.

To further illustrate, one embodiment of the present invention involvesthe addition of liquid mercury sorbent containing calcium bromide andwater directly to raw or crushed coal prior to combustion. Addition ofliquid mercury sorbent containing calcium bromide ranges from 0.1 to 5%,preferably from 0.025 to 2.5% on a wet basis, calculated assuming thecalcium bromide is about 50% by weight of the sorbent. In a typicalembodiment, approximately 1% of liquid sorbent containing 50% calciumbromide is added onto the coal prior to combustion.

In another embodiment, the invention involves the addition of calciumbromide solution both directly to the fuel and also in a zone of thefurnace characterized by a temperature in the range of 2200° F. to 1500°F. In this embodiment, liquid mercury sorbent is added both beforecombustion and after combustion. Preferred treat levels of calciumbromide can be divided between the pre-combustion and post-combustionaddition in any proportion.

In another embodiment, the invention provides for an addition of acalcium bromide solution such as discussed above, solely into thegaseous stream in a zone of the furnace characterized by a temperaturein the range of 2200° F. to 1500° F.

The invention has been described above with respect to various preferredembodiments. Further non-limiting disclosure of the invention isprovided in the Examples that follow. They illustrate the effectivenessof the invention when a liquid only and a liquid/solid sorbent system isapplied for mercury remediation of fuels.

EXAMPLES

In the Examples, coals of varying BTU value, sulfur, and mercury contentare burned in the CTF furnace at the Energy Environmental ResearchCenter (EERC) at the University of North Dakota. Percent mercury andsulfur reductions are reported based on the total amount of the elementin the coal prior to combustion.

Example 1

This example illustrates the mercury sorption ability of a calciumbromide/water solution when applied to a Powder River basinsub-bituminous coal. The as-fired coal has a moisture content of 2.408%,ash content of 4.83%, sulfur content of 0.29%, a heating value of 8,999BTU and a mercury content of 0.122 μg/g. Combustion without sorbentresults in a mercury concentration of 13.9 μg/m³ in the exhaust gas. Thefuel is ground to 70% passing 200 mesh and blended with 6% of a sorbentpowder and 0.5% of a sorbent liquid, based on the weight of the coal.The powder contains by weight 40-45% Portland cement, 40-45% calciumoxide, and the remainder calcium or sodium montmorillonite. The liquidis a 50% by weight solution of calcium bromide in water.

The sorbents are mixed directly with the fuel for three minutes and thenstored for combustion. The treated coal is fed to the furnace.Combustion results in a 90% mercury (total) removal at the bag houseoutlet and a 80% removal of sulfur as measured at the bag house outlet.

Example 2

This example illustrates the use of powder and liquid sorbents appliedto three bituminous coals of varying mercury content. All coals areprepared as in Example #1, with the same addition levels of sorbents.

% of Mercury % Sulfur Parameter Coal Removal Removal % Moisture    8.48Pittsburgh, 97.97 40.0 % Sulfur    2.28 Seam, Bailey Mercury 16.2 μg/m³Coal BTU value 13,324   % Moisture    10.46 Freeman Crown 97.9  36.0 %Sulfur    4.24 III Mercury 8.53 μg/m³ BTU value 11,824   % Moisture   1.0  Kentucky Blend 90.1  52.0 % Sulfur    1.25 Mercury 5.26 μg/m³BTU value 12,937  

Example 3

This example illustrates addition of a mercury sorbent post-combustion.Pittsburgh Seam-Bailey Coal is ground to 70% passing 200 mesh. Nosorbent was added to the fuel pre-combustion. Liquid sorbent containing50% calcium bromide in water is duct injected into the gaseous stream ofthe furnace in the 2200° F.-1500° F. zone. The liquid sorbent isinjected at the rate of approximately 1.5% by weight of the coal.

Sorbent Coal Type Composition % S reduction # Hg Reduction PittsburghSeam- 50% CaBr₂ 28.13 96.0 Bailey Coal 50% H20

Example 4

This example illustrates addition of a liquid and a powder sorbentpost-combustion. No sorbent was added directly to the fuel. Both fuelsare bituminous and noted as Freeman Crown III and Pittsburgh Seam—BaileyCoal. In both cases the coal was ground to 70% minus 200 mesh prior tocombustion. The powder and liquid sorbents are as used in Example 1.Rates of liquid and powder addition (percentages based on the weight ofthe coal being burned), as well as mercury and sulfur reduction levels,are presented in the table.

Powder Liquid sorbent sorbent S Hg Coal Type injection rate injectionrate Reduction Reduction Freeman Crown 1.0 4.0  36.27 97.89 IIIPittsburgh Seam- 1.5 6.10 33.90 96.00 Bailey Coal

Example 5

Pittsburgh Seam Bailey Coal is prepared as in Example 1. The powdersorbent of Example 1 is added to the coal pre-combustion at 9.5% byweight. The liquid sorbent of Example 1 (50% calcium bromide in water)is injected post-combustion in the 1500° F.-2200° F. zone at a rate of0.77%, based on the burn rate of the coal. Sulfur reduction is 56.89%and mercury reduction is 93.67%.

Example 6

Kentucky Blend Coal is prepared as in Example 1. The powder sorbent ofExample 1 is added to the coal pre-combustion at 6% by weight. Theliquid sorbent of Example 1 (50% calcium bromide in water) is injectedpost-combustion in the 1500° F.-2200° F. zone at a rate of 2.63%, basedon the burn rate of the coal. Sulfur reduction is 54.91% and mercuryreduction is 93.0%.

Although the invention has been set forth above with an enablingdescription, it is to be understood that the invention is not limited tothe disclosed embodiments. Variations and modifications that would occurto the person of skill in the art upon reading the description are alsowithin the scope of the invention, which is defined in the appendedclaims.

What is claimed is:
 1. A method for reducing emissions of mercury andsulfur arising from combustion of coal in the furnace of a coal burningsystem, comprising adding a sulfur sorbent composition and a mercurysorbent composition into the coal burning system, wherein the mercurysorbent composition comprises a bromine compound, and the sulfur sorbentcomposition comprises cement kiln dust.
 2. A method according to claim1, wherein the mercury sorbent composition comprises calcium bromide. 3.A method according to claim 1, wherein the sulfur sorbent compositioncomprises cement kiln dust.
 4. A method according to claim 1, whereinthe sulfur sorbent composition comprises greater than 2% by weightsilica and greater than 2% by weight alumina.
 5. A method according toclaim 1, wherein the sulfur sorbent composition further comprises analuminosilicate clay.
 6. A method according to claim 1, wherein 90% ormore of the mercury in the coal is captured in the ash product producedfrom combustion.
 7. A method according to claim 1, comprising adding themercury sorbent onto the coal before combustion in the furnace.
 8. Amethod according to claim 1, wherein the sorbent compositions are addedseparately into the coal burning system.
 9. A method for reducing theamount of mercury released into the atmosphere from burning coal in acoal burning system, comprising adding a first sorbent comprising ahalogen and a second sorbent composition comprising calcium and furthercomprising greater than 2% by weight silica and greater than 2% byweight alumina into the coal burning system.
 10. A method according toclaim 9, wherein the first sorbent comprises a bromine compound.
 11. Amethod according to claim 9, wherein the second sorbent comprises cementkiln dust.
 12. A method according to claim 11, wherein the secondsorbent further comprises MgO.
 13. A method according to claim 9,wherein the second sorbent comprises cement kiln dust and analuminosilicate clay.
 14. A method according to claim 9, comprisingadding the first and second sorbents separately into the coal burningsystem.
 15. A method according to claim 9, comprising adding the firstsorbent onto the coal before combustion.
 16. A method according to claim9, wherein more than 70% by weight of the mercury in the coal iscaptured in the ash produced from combustion of the coal.